1. Field of the Invention
The present invention relates to a thermosensitive recording material and, more particularly, to a thermosensitive recording material comprising a leuco dye which is colorless or light colored at room temperature, serving as a coloring agent, and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto.
2. Discussion of Background
In the field of information recording, as a great variety of information is increasing, many kinds of recording materials are studied, and some are put to practical use to meet the demands for energy-saving and non-pollution.
In particular, a thermosensitive recording material is widely utilized in a variety of fields, such as in the printer for computers and electronic calculators, the recorder for medical measuring instruments, high-speed facsimile apparatus, ticket vending apparatus and thermosensitive copying apparatus because it has the following advantages:
(1) Color development can readily take place in the thermosensitive recording material only by application of heat thereto and clear images are formed thereon without any complicated development process.
(2) The thermosensitive recording material can be manufactured using a relatively simple and compact installation. In addition, the thermosensitive recording material is convenient to handle and the maintenance cost thereof is low.
(3) In general, a sheet of paper is used as a substrate of the thermosensitive recording material. Therefore, not only the cost of the substrate of the recording material can be decreased, but also the touch of the thermosensitive recording material is close to that of plain paper.
The above-mentioned thermosensitive recording material is prepared by coating on a substrate such as a sheet of plain paper, a sheet of synthetic paper and a synthetic resin film a coating solution which contains coloring components capable of causing the coloring reaction under application of heat thereto, and then drying it. When the thermal energy is applied to the thermosensitive recording material with a thermal head or thermal pen, the above coloring components induce the coloring reaction, so that images can be formed on the thermosensitive recording material.
Conventionally, many thermosensitive recording materials are proposed, for example, in Japanese Patent Publications 43-4160 and 45-14039. These conventional thermosensitive recording materials, however, are poor in the coloring thermosensitivity, so that the images with high image density cannot be obtained in the high-speed recording.
To solve the abovementioned problem, the leuco dyes of 3-N-methyl-N-cyclohexylamino-6-methyl-7-anilinofluoran and 3-dibutylamino-6-methyl-7-anilinofluoran are used as the coloring agent with improved coloring sensitivity, as disclosed in Japanese Laid-Open patent application nos. 49-109120 and 59-190891, respectively. In addition, the use of color developers with improved coloring performance, for instance, 1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane (Japanese Laid-Open patent application no. 59-106456) and 1,5-bis(4-hydroxyphenylthio)-3-oxapentane (Japanese Laid-Open patent application no. 59-116262) is proposed. In Japanese Laid-Open patent application nos. 61-123584, 61-215087 and 61-242889, the above leuco dyes and color developers are used in combination to realize the high-speed recording with high coloring thermosensitivity.
However, the coloring agent with rapid color development has the drawback that the fade-out readily occurs. This has an adverse influence on the reliability of the thermosensitive recording material.
To prevent the fade-out of the color image, anti-oxidizing agents, such as 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl) butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl) butane, 4,4'-thiobis(6-tert-butyl-2-methyl) phenol, 4-benzyloxy-4'-hydroxyphenylsulfone, tetrabromobisphenol A and tetrabromobisphenol S, are usually added to the coloring components. However, these anti-oxidizing agents induce the fogging on the background of the recording material, or the background thereof tends to yellow. Furthermore, the coloring thermosensitivity is decreased when the above anti-oxidizing agent is added to the coloring components.
As disclosed in Japanese Laid-Open patent application nos. 59-187890, 59-152892 and 60-210490, 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethyl) isocyanuric acid is used together with the coloring components. However, the combinations shown in the above applications do not contribute to the improvement in the high coloring thermosensitivity of the thermosensitive recording material and high-speed recording thereof.